The present invention relates to a method for injection molding plastics materials under low shear conditions in the forward portion of the barrel. More specifically, the invention relates to a methods for producing articles having a mottled appearance, and for processing glass reinforced materials.
In order to produce mottled plastic articles having streaks of diverse colors or shades so as to simulate the appearance of marble or wood, two or more materials having different colors may be processed simultaneously in an injection molding machine. Although the materials must be softened sufficiently to enable them to be molded, they must not be blended to the point that the diverse colors are no longer distinguishable in the end product. In many cases, the different materials have different melt viscosities so that there results a very poor distribution of the materials as they are melted, and this produces a streaked finish appearance. Although the different viscosities impede good mixing, if the materials undergo a sufficient amount of shear, they will be mixed to the point that the final color will be a combination of the two starting colors and there will be no mottled appearance whatsoever.
An early injection molding technique which was satisfactory for producing mottled articles comprises an injector unit of the type having a barrel and plunger disposed therein for reciprocating movement. The barrel is heated, as by heating bands which circle the outer periphery thereof, and by inserting molding materials of different colors ahead of the plunger and then advancing the plunger, the materials are displaced toward the discharge orifice of the barrel. Contact with the heated surface of the barrel causes the materials to soften so that they are capable of conforming to the molding surfaces. Since this type of apparatus does not perform much mixing on the material, it was necessary to pre-blend the material before introducng them into the barrel.
The type of injection molding apparatus in wide use today comprises an elongated barrel having a screw which extends longitudinally therein. The screw has a helical land on its surface which moves relative to the internal surface of the barrel so as to work the plastics material and feed it toward the outlet end of the barrel. In order to inject the material, the screw is advanced forwardly toward the discharge opening so as to force the softened material into the mold cavity.
One characteristic of screw-type injection molding machines is that a high degree of shear energy is imparted to the material so that intense mixing occurs during feeding as the screw rotates. In the case of processing materials having diverse colorations so as to produce a mottled product, this intense mixing completely blends the two materials so that distinct color patterns, such as streaks, are destroyed.
Conventional preciprocating screw-type injection molding machines can be utilized for molding mottled products, but it is necessary to use a screw which induces minimal mixing of the molten plastic. This can be achieved by using a short screw having a length to diameter ratio (L/D) of 13:1, for example, but in the normal situation where an injection molding machine is used for other applications which require a longer barrel, such as a barrel having an L/D of 26:1, economic considerations often do not warrant the expense of purchasing a separate barrel just to run mottled parts. It would be advantageous, therefore, to have the flexibility to run both mottled parts and parts requiring a longer L/D barrel without the necessity of stocking two barrels and completely converting the machine when switching from one to the other.
One solution to the problem of enabling mottled parts to be run with a conventional barrel is disclosed in U.S. Pat. No. 3,945,786, which is owned by the assignee herein. This injection molding machine comprises a conventional length barrel having a screw therein which is disposed in the rear portion of the barrel so that a substantial distance is present between its tip and the discharge opening. A spreader device is positioned within a heated portion of the bore ahead of the feed screw, and as the screw is rotated to feed and soften molding materials of diverse coloration and is subsequently translated forwardly, the softened and partially blended materials are pushed through the spreader device such that final plasticization occurs. Since the length of the flighted, rotating screw is relatively short, the materials of diverse coloration are discharged from the screw in a softened condition, but not blended to the point that the distinct color patterns are lost.
One problem with the mottling apparatus disclosed in U.S. Pat. No. 3,945,786 is that the conventional barrel is designed such that it will hold full injection pressure within defined safety limits only in the front portion thereof. When the shortened screw retracts rearwardly of the portion of the barrel designed to hold full injection pressure, the injection pressure therein during forward translation of the screw would exceed safety limits and may be sufficient to burst the barrel. This required the use of a longer screw than was optimum.
The present invention is also applicable to the molding of glass reinforced materials, such as BMC, where breakage of the glass fibers is a problem when excessive shear is performed on the material. Although a certain degree of shear is inevitable if the material is to be conveyed forwardly by the rotating screw, if a low shear environment can be provided for the material after sufficient pumping pressure has been developed, then an improved product would result. As in the case of mottling, the use of a short screw would tend to minimize shear, but the excessive pressures within the intermediate portion of the barrel during the injection stroke would cause the barrel to burst.